1. Field of the Invention
The present invention relates to a process for preparing 2,2-bis(4'-hydroxyphenyl)propane (hereinafter referred to as bisphenol A) through dehydration-condensation of phenol and acetone.
Bisphenol A is an extremely useful compound for industrial purposes and is generally used as an important material for the production of polycarbonates, polyesters, epoxy resins, and developers of heat-sensitive paper. Bisphenol-A applied to polycarbonate resins is required to be colorless and of high purity.
2. Description of the Related Art
Bisphenol A is typically prepared by dehydration-condensation of two molecules of phenol and one molecule of acetone in the presence of an acid catalyst like hydrochloric acid.
When the reaction is carried out at a relatively low temperature using hydrochloric acid as a homogeneous catalyst, it proceeds with crystallization of an adduct of phenol with bisphenol A, thereby giving bisphenol A with a high conversion of acetone and a high 4,4'-selectivity. The homogeneous acid catalyst like hydrochloric acid, however, requires removal of the catalyst from a reaction mixture or neutralization of the reaction mixture. Therefore, the process is so complicated as to consume much time and labor. Homogeneous dissolution of the acid in the reaction solution further causes corrosion of equipment used in the reaction. Reaction vessels should be made of rather expensive, anti-corrosive materials accordingly.
In view of such circumstances, solid heterogeneous catalysts have increasingly been applied to the industrial preparation of bisphenol A.
Known examples of solid heterogeneous catalysts include zeolites, salts of heteropolyacids partially neutralized and insolubilized, and strongly acidic cation exchange resins. These solid heterogeneous catalysts, however, have relatively low catalytic activity and selectivity.
A method applicable to overcome the low performance of these solid catalysts is to add a sulfur-containing compound as a cocatalyst together with an acid catalyst to the reaction system. It is known that addition of such cocatalyst enhances the catalytic activity and reaction selectivity.
Known sulfur-containing compounds exerting the enhancement effect are thiol compounds like alkylmercaptan and benzylmercaptan. These compounds are homogeneously dissolved in the reaction system to enhance the activity of the acid catalyst and realize the high 4,4'-selectivity of bisphenol A, but they cause the problem of purification of the final product or bisphenol A; that is, bisphenol A should be separated from these cocatalysts like the homogeneous acid catalyst described above.
Several methods have been proposed to fix these thiol compounds and to prevent contamination of the final product with the thiol compounds. A method disclosed in Japanese Patent Publication No. 37-14721, for example, esterifies a mercaptoalkyl alcohol with part of acidic groups of a strongly acidic cation exchange resin and fixes the mercapto compound to the cation exchange resin through the ester linkage. Another method disclosed in Japanese Patent Publication No. 46-19953 partially neutralizes a strongly acidic cation exchange resin with a mercaptoalkylamine to fix the mercapto compound. A method disclosed in Japanese Patent Laid-open No. 52-19189 partially neutralizes a strongly acidic cation exchange resin with a cyclic mercaptoamine to fix the mercapto compound through ionic bonding. Another method disclosed in U.K. Patent No. 1539186 fixes a mercaptoamino acid through ionic bonding with a cation exchange resin. The catalysts prepared by fixing mercapto compounds to ion exchange resins, however, easily deteriorate due to the low heat resistance of the ion exchange resins. Mercapto compounds fixed by the above processes are thermally unstable and easily decomposed and liberated. This results in similar drawbacks of the homogeneous acid catalysts and homogeneous mercapto compounds described above. The fixation is attained by the reaction of the mercapto compounds described above with acid groups which can effectively work as reaction catalyst, thereby decreasing the quantity of effective acid.